X-ray mammography is the current clinical tool used for breast cancer detection. However, mammography is ionizing radiation and has unacceptable false negative rate for patients with radiodense breast tissues. These patients represent the general population of premenopausal women as well as those with fibrocystic tissue disease. Yet, cancer in younger women tends to be more virulent and grow faster. Consequently, there is a critical need to seek alternatives that could overcome the problems associated with x-ray mammography and that could become an excellent adjunct or competitive tool to conventional x-ray mammography. Non-ionizing, noninvasive near-infrared optical imaging has great potential to become such promising alternative for breast cancer detection. Based on the very recent development in this regard, we have identified an optical imaging method that combines both modern optical techniques and powerful computations. Our pilot clinical studies have shown that breast tumors can be detected using optical imaging, which indicates the possibility of clinical application of this technique for breast imaging and breast lesion detection characterization. Our specific aims in this project include (I) continued development of both two- and three-dimensional computer algorithms needed for generating images, (2) construction and test of an improved optical imaging system using three-dimensional tomographic data collection, (3) evaluation and optimization of this system using tissue phantom experiments, and (4) test of this optical imaging system in human subjects. We believe that the success of this research will provide solid evidence that simple steady-state, continuous-wave optical methods coupled with model-based image formation from tomographically measured data offers an imaging methodology with clinical relevance to the breast cancer detection.